In the related art, internal combustion engines are proposed in which the shape of a spark plug is devised. For example, JP 2015-76193 A discloses a structure in which the spark plug has a bent shape such that an extension portion of a ground electrode of the spark plug extends into a combustion chamber, and a tip of the extension portion is directed toward a tip of the central electrode. In this structure, a through hole which is opened toward a gap between the tip of the central electrode and the tip of the ground electrode is formed on a wall surface of a tip portion of a housing near a base end of the ground electrode, and the through hole is inclined toward the gap between the electrodes with respect to a central axis direction of the spark plug. With such a structure, contact of the spark formed by electrical discharge of the spark plug with an upper wall surface of the combustion chamber can be suppressed.
JP 2015-124674 A discloses a structure in which the central electrode is provided at a tip of a ceramic insulator of the spark plug, and the ground electrode extends from the housing into the combustion chamber. A direction A of an air flow generated around the central electrode and the ground electrode upon ignition of the spark plug and a direction B of a connection portion of the central electrode with respect to the housing of the ground electrode as seen from the central electrode intersect each other. At a tip portion of the ground electrode, an inclined surface which is inclined to a side opposite a ceiling surface of the combustion chamber toward a downstream side of the direction A of the air flow is formed at a side opposite the central electrode. With such a configuration, contact of an initial flame generated from the electrical discharge spark with the ceiling surface of the combustion chamber is suppressed.
JP 2007-323903 A discloses a structure in which the spark plug includes a ground electrode which extends from an end of a tubular housing into the combustion chamber, and which forms an electrical discharge gap between the central electrode and the ground electrode. In the combustion chamber, a through hole is formed at the end of the housing. With such a configuration, the air flow in the combustion chamber enters inside the housing through the through hole, and flows from a region near the electrical discharge gap toward the inside of the combustion chamber, and thus, generation of the electrical discharge at a position different from the electrical discharge gap can be suppressed.
None of JP 2015-76193 A, JP 2015-124674 A, and JP 2007-323903 A discloses a structure in which a trailing vortex is created by the spark plug, and is used for flame holding which is a stable holding of the frame by ignition, to thereby promote and stabilize combustion. For example, the techniques of JP 2015-76193 A and JP 2015-124674 A are directed to suppressing contact of the spark or the flame to the upper wall surface of the combustion chamber. In the techniques of JP 2015-76193 A and JP 2015-124674 A, the spark and flame are sent to the downstream by the air flow, and the trailing vortex of the electrode is not used for flame holding.
The technique of JP 2007-323903 A is directed to suppressing generation of electrical discharge at a position different from the electrical discharge gap by blowing the spark between the electrical discharge gap by the air flow. In this technique, the through hole is merely used to set the direction of the air flow between the electrodes to be to the opposite side from the central electrode, and the trailing vortex of the electrode is not used for flame holding.
In the spark plug of the related art, the flame holding at the spark plug is not done under a flame-resisting condition such as high flow rate of the air flow and dilute combustion (lean A/F, EGR dilution), or the like, and high energy is necessary for stable ignition.